Vehicle guidance system

ABSTRACT

A system for guiding a vehicle on a surface along a predetermined path defined by at least one wire coupled to an oscillator for generating an electromagnetic field. A field sensor carried by the vehicle including pick-up coils having mutually orthogonal axes lying in a plane substantially parallel to the plane of the track senses the horizontal components of the field. Signals induced in the pick-up coils are then utilized by a deviation computer in the vehicle to determine the angle and distance of deviation from the track. A steering control which responds to the deviation adjusts the steering mechanism of the vehicle to minimize the deviation.

Moore et al.

[ VEHICLE GUIDANCE SYSTEM Primary ExaminerKenneth l-l. Betts AssistantExaminer-John P. Silverstrim AttorneyWoodcock, Washburn, Kurtz &Mackiewicz [57] ABSTRACT A system for guiding a vehicle on a surfacealong a predetermined path defined by at least one wire coupled to anoscillator for generating an electromagnetic field. A field sensorcarried by the vehicle including pick-up coils having mutuallyorthogonal axes lying in a plane substantially parallelto the plane ofthe track senses the horizontal components of the field. Signals inducedin the pick-up coils are then utilized by a deviation computer in thevehicle to determine the angle and distance of deviation from the track.A steering control which responds to the deviation adjusts the steeringmechanism of the vehicle to minimize the deviation.

15 Claims, 6 Drawing Figures RIGHT [75] Inventors: Wilcy I. Moore,Folsom, Pa.; Gerard L. Lafond, Cinnaminson, NJ. [73] Assignee: VersadyneInc., Prospect Park, Pa.

[22] Filed: July 9, 1971 [21] Appl. No.: 161,078

[52] U.S. Cl. 180/98, 318/587 [51 860k 27/06 [58] Field of Search180/98, 79.1, 79; 318/580, 587

[56] References Cited UNlTED STATES PATENTS 3,009,525 11/1961 DeLiban180/98 2,990,902 7/1961 Cataldo l80/79.l X

3,612,206 10/1971 Ohntrup 180/98 3,338,328 8/1967 Cataldo ISO/79.13,132,710 5/1964 Petrella et a1 180/98 X NOSEWHEEL AXIS AIRCRAFT AXISFIELD DIRECTION LEFT PATENTEIJSEPI nan 757. 887

sum 1 0F 4 SIGNAL l4 16-2 /;%A{ SOUFCE Is \FIELD SENSOR l2 PARALLAXCORRECTION CONTROLS DEVIATION COMPUTER OPERATOR DISPLAYS GUIDANCECOEFFICIENTS I STEERING FEEDBACK 24 STEERING MECHANISM NOSEWHEEL FIELDDIRECTION Jig Z PAIENTEDSEPI 1' um I saw u or 4 VEHICLE GUIDANCE SYSTEMBACKGROUND OF THE INVENTION This invention relates to systems forguiding a vehicle on a surface along a predetermined path.

More particularly, this invention relates to the vehicle guidance systemhaving a track including at least one wire carrying a field generatingcurrent where the wire substantially coincides with the predeterminedpath. In such a system, field sensing means such as pick-up coils arecarried by the vehicle with the field inducing signals indicative of thevehicle deviation from the predetermined path. Through the use ofsuitable servo means, corrective steering measures are taken to minimizethe deviation and bring the vehicle back to the predetermined path.

PRIOR ART ln vehicle ground guidance systems, both a single as well as apair of track wires have been utilized. U.S. Pat. No. 2,990,902 Cataldodiscloses a system of the single wire type and utilizes a pair ofvehicle mounted coils for sensing deviation from a path coincident withthe guidance wire. As disclosed in this patent, the coils are inclinedwith respect to a plane parallel to the single track wire.

A single wire vehicle guidance system is also disclosed in U.S. Pat. No.3,009,525 De Liban wherein three mutually orthogonal sensing coils aremounted on the vehicle utilized. In the De Liban system, the lateraldisplacement from the single wire track is determined by sensing avertical field. Such an arrangement is particularly unsatisfactory in anaircraft due to the presence of a highly conductive horizontal surfacein the form of the aircraft skin which adversely effects any sensing ofthe vertical field near the skin of the aircraft. The determination oflateral displacement in the De Liban system is undesirably affected bythe vehicle angle with respect to the track rather than beingindependent of the vehicle angle. Furthermore, this system does notsense a vehicle angle but rather the steering angle of the vehicle.

U.S. Pat. No. 3,132,710 Petrella et al. discloses a vehicle groundguidance system having a pair of track wires carrying signals ofopposite phase. The deviation of the vehicle from a path defined by thetrack wires is sensed by a pair of coils, one horizontal and onevertical. Because one of the coils is vertical, the coils must belocated on the nose wheel of any aircraft utilizing the system in orderto minimize the efi'ect of the aircraft skin on the vertical componentsof the field generated by the track wires. However, the mounting of thecoils on the nose wheel is undesirable in that ,it creates substantialproblems because of the abuse the coils take at this location.

The system of the Petrella patent does not determine and therefore doesnot rely upon critical information such as the angle between thepredetermined path and the aircraft longitudinal axis or the nose wheeldirection. It is not therefore capable of determining the correct nosewheel angle independnet of the instantaneous nose wheel angle andposition of the aircraft with respect to the path. Furthermore, it isnot capable of optimizing the corrective approach angle of the aircraftto the proper path which is a function of the maximum allowable aircraftvelocity.

SUMMARY OF THE INVENTION In accordance with one aspect of the invention,a vehicle guidance system of the track wire type is provided whichrelies exclusively on the horizontal components of the track field. Thisaspect of the invention involves the use of a pair of pick-up coilshaving mutually orthogonal axes parallel to the track surface whichsense the horizontal components of the fields. The pick-up coils maythen be mounted adjacent the skin of a vehicle such as an aircraftwithout detrimentally affecting the sensed horizontal components of thefields. Furthermore, the pick-up coils are not subjected to the abusewhich is characteristic of a nose wheel mounting position.

In accordance with another aspect of the invention, the correct steeredpath is determined independently of the instantaneous steered path andthe position of the vehicle with respect to the predetermined path. Thisaspect of the invention is provided by a deviation computer which solvesthe equation 0,, 0,, 4a where (b represents the angle between theinstantaneous steered path of the steering wheel and the longitudinalaxis of the vehicle. 0,, represents the angle between the predeterminedpath and the steered path and the angle 0,, represents the angle betweenthe predetermined path and the longitudinal axis of the vehicle. Theangle qb is determined by a steering control means and applied to theinput to the deviation computer for solution of the steering equation.Both the angle 0,, and 0,, are determined from the signals induced inthe pair of pick-up coils. The output of the deviation computerrepresents angular deviation in the form of an error signal which isapplied to the-steering control means to correct the steered path.

In accordance with still another aspect of the invention, steeringcorrections are made at an optimum angle. This aspect of the inventionis provided by the deviation computer which determines the angle 0,,from the expression IK DI IK D I where D represents the distance betweenthe steering wheel means and the predetermined path, K 8 Lyn/V and Kl/V,,, where is the maximum turning rate of the steering wheel means andV,, is the maximum velocity of the vehicle. In accordance with anotheraspect of the invention, the signal is generated indicating a certainthreshold distancehas not been exceeded. In an embodiment of theinvention utilizing a single track wire, an additional pick-up coilhaving 'an axis perpendicular to the longitudinal axis of the vehicle isprovided. Utilizing the signals induced in the additional pick-up coiland one of the pair of pick-up coils perpendicular to the longitudinalaxis of the vehicle, a field gradient is determined by the deviationcomputer which is utilized in generating the position indicating signal.The field gradient is also utilized for normalization purposes incomputing the angles 6,, and 6,. In a two track wire embodiment of theinvention, the pick-up coil having an axis transverse to thelongitudinal axis of the vehicle provides a signal to the deviationcomputer for determining the position of the vehicle with respect to thepredetermined path.

In accordance with another aspect of the invention, the lateraldisplacement of the vehicle from the track is independent of the vehicleangle. This is true for both the single wire track system and the twotrack wire system.

These and other aspects of the invention are more fully explained in thefollowing specification which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially schematic blockdiagram of the basic elements in a system embodying the invention;

FIG. 2 provides a graphical explanation of the guidance nomenclatureutilized in describing the invention;

FIG. 3 is a schematic diagram of the ground track subsystem in oneembodiment of the invention;

FIG. 4 is a schematic diagram of the vehicle subsystem in the embodimentof FIG. 3;

FIG. 5 is a schematic diagram of a ground track subsystem in anotherembodiment of the invention; and

FIG. 6 is a schematic diagram of the vehicle subsystem in the embodimentof FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The basic elements of asystem embodying various aspects of the invention and capable of guidinga vehicle along a predetermined path or track will now be described withreference to FIG. 1. As shown, the track 10 which may comprise one ormore wires or cables is coupled to and supplied by a signal source 12and defines a predetermined path axis of travel 14 for a vehicle underthe control of the guidance system. The horizontal components of thefield generated by the signal source 12 and the wires of the track 10are sensed by a field sensing means 16 including a pair of pick-up coils16-1 and 2 having mutually orthogonal axes with the axis of one of thecoils being parallel to the longitudinal axis of the vehicle carryingthe pick-up coils 16. The signals which are induced in the pick-up coilsare then applied to a deviation computer 18 which is also carried by thevehicle for determining both the angular deviation of the vehicle fromthe path 14 as well as the displacement or position of deviation fromthe path 14.

In addition to input in the form of signals induced in the coils 16, thedeviation computer 18 also receives a number of additional inputs aswell as providing two outputs. Parallax correction which willbe-described in somewhat further detail is provided in order to correctfor the difference in displacement of the pick-up coils from the path 14as compared with the displacement of the steering wheel(s) from the path14. Guidance coefficients such as V and (1a are also input to thedeviation computer 18 for purposes of optimizing the corrective steeredpath.

The output of deviation computer 18 in the form of a deviation or errorsignal is applied to a guided steering control means 20 which comprisessuitable servo means. The output of thc guidance steering control 20which represents the angle of the steering wheel(s) in then applied as afeedback signal to the deviation computer 18.

The output of an actuator 20 is connected to a steering mechanism 24which may include a steering wheel. A clutch may be provided todisconnect the output of the actuator from the steering mechanism 24.

In the foregoing discussion, the term vehicle has been utilized as ageneric term for any apparatus which is intended to be steered in apredetermined path across a surface. As such, the term vehicle isintended to embrace an aircraft taxiing along the ground. Since theinvention has particular utility in a taxi guidance system for aircraft,such systems will now be described in substantial detail and referencewill now be made to FIG. 2 for purposes of explaining the nomenclatureassociated with an aircraft taxi guidance system.

As shown in FIG. 2, the steering mechanism 24 comprises a steering nosewheel(s) of an aircraft having a longitudinal axis parallel to one ofthe pick-up coils 16-1 and orthogonal to the other pick-up coil 16-2. Anumber of angles, all of which are computed by the deviation computer 18of FIG. 1, are defined by the angles between the longitudinal axis 26 ofthe aircraft, the predetermined path 14, and the nose wheel axis or theinstantaneous steered path 28. The angle represents the angle betweenthe longitudinal axis 26 and the nose wheel axis 28. The angle 0,,represents the angle between the nose wheel axis 28 and the longitudinalpath axis 14. Finally, the angle 0,, represents the angle between thelongitudinal axis 26 and the predetermined path axis 14.

Another important variable which is determined by the deviation computer18 is the distance D which represents displacement or position deviationfrom the predetermined path axis 14. This displacement or positiondeviation is designated as D, when referring to the position of thepick-up coils 16. The displacement or position deviation is designated Dwhen referring to the position of a wheel strut 30 in the steeringmechanism 24. For displacement or position deviation to the right of thepredetermined path axis 14, the values of D, both D, and D,,,, arepositive. For displacement or position deviation to the left of thepredetermined path axis 14, the values of D and'D are negative. Althoughthe difference of D and 0,, may seem rather small, the difference doesbecome quite significant for large values of X which represents thedistance between the point of intersection for the coils 16-1 and 2 andthe strut 30. Large values of X and values 0,, unequal to zero will leadto some parallax error and accordingly, the value of X becomessignificant in parallax correction as will be subsequently described.

A two wire track embodiment of the invention for an aircraft taxi systemwill now be described with reference to FIGS. 3 and 4. Describing firstthe ground track sub-system shown in FIG. 3, the signal source 12comprises an oscillator 32 generating a signal F, having acharacteristic frequency f which may be of the order 500 Hz and afrequency multiplier 34 for obtaining a signal F having a characteristicfrequency f which may be of the order of 1,000 Hz. The signals F and Fare summed at a summing amplifier 36 to obtain a signalIFI1SII'lw1t'lI2SII'Iw2t where w1=21If and (02 2 llfz. This signal isthen amplified in apower amplifier 38 and applied to the right wire ortrack leg 40 of the two wire track. The same signals are also applied toa differential amplifier 42, amplified at a power amplifier 44, andapplied as 1,, I,sinw,t l sinm t to the left wire or track leg 46. Ashielded wire 48 completes the circuit for the wires 40 and 46.

The signal f generates a relatively constant level field, while thefield signal from f generates a ramp.

The horizontal field components, which are generated by the subsystem ofFIG. 3, are sensed and operated upon by the vehicle subsystem shown inFIG. 4. The subsystem comprises the field sensor 16, the deviationcomputer 18, the guided steering control means 20, and the steeringmechanism 24.

As shown, the field sensor 16 comprises the coils 16-1 and 2. Since thepick-up coil 16-1 intersects the predetermined path axis 14 at an angle0,, a signal including an EF sin 0, component is induced in coil 16-1while a signal including a component E,F,cos0, E F Kds cos 0,, isinduced in the coil 16-2. The signals which are induced in the coils16-1 and 2 by the field are now applied to the deviation computer 18.

After amplification by amplifiers S0 and 52, the signals are applied tofilters 54, 56, and 58. The filter 54 which is coupled to the output ofthe amplifier 50, passes the component having the characteristicfrequency f while blocking the components having the characteristicfrequency f to obtain an output E,F sin 0,. The filters 56 and 58 whichare parallel and coupled to the output of the amplifier 52 only passfrequencies f, and f respectively so as to obtain two separate outputsignals E,F cos 0, and E F KDs cos 0,.

The signal E,F, sin 0, is now demodulated at a demodulator 60 which iscoupled to a synchronized oscillator 62 having a characteristicfrequency f to obtain a signal representing the K sin 0, where K is aconstant. Since 0, is usually a small angle, the sin 0, is a very goodapproximation of the angle 0,, so that the output of the demodulator 60provides a signal which is a very close approximation of the quantityK0,.

The signal representing E F, cos 0, is rectified at a rectifier 64 andapplied to a low pass filter 66 to produce the signal E cos 0, at theinput to a divider 68. The signal representing the signal E F KDs cos 0,is demodulated at a demodulator 70 using the second harmonic of fgenerated by the oscillator 62 after rectification at a rectifier 72 andfiltering at a filter 74 passing the frequency f After demodulation, asignal representing E KDs cos 0, is applied as another input to thedivider 68.

Since the signal representing E,F cos 0, will not go to zero undernormal guidance conditions, the signal may be utilized to synchronizethe oscillator 62. Similarly, the signal representing E, cos 0, may alsobe used as an input to an amplifier 76 along with an in track" thresholdsignal. Amplifier 76 compares the E cos 0, signal with the in trackthreshold signal to generate an in track signal for display in thecockpit of the aircraft.

The E KDs cos 0, signal and the E cos 0, signal are divided at thedivider 68 to produce a signal representing (E KDs Cos 0,)/(E, Cos 0,) EKDs/E, K,D,. It will be noted that division by the function Cos 0,eliminates the effect of vehicle angle on the determination of vehicledisplacement D,. After obtaining the K,D, signal from the divider 68,the signal is subtracted from an X0 signal at a differential amplifier78 to obtain an output signal reprsenting D,,,. The X0, signal isobtained from the tap of a potentiometer 80 which is connected acrossthe signal K0,. By appropriately setting the potentiometer 80 so as tocorrespond to the value X/K, the signal X0, is obtained. The 0,, signalis then modified by boundary coefficient K, to obtain the quantity value|K,D,,,l while the quantity K 1), is obtained from a derivativeamplifier circuit 82. Subtraction of the K 0 signal from the |K,D,,l isperformed at an amplifier 84 to obtain the arigle 0;,

Having now determined signals representing 0,, and the angle 0,, andutilizing a signal representing the 11 obtained from a slide wire 86 ofthe steering control means 20, the deviation computer 18 solves thesteering equation 42 0,, 0, or 0,, 0,, 0 in the following manner. Thesignals 0,, 0,, and q5 are applied as inputs to a summing amplifier 88which generates a deviation or error signal representing the deviationor error from the predetermined path axis 14. Upon application of thedeviation signal to a servo actuator 90 of the steering control means20, the servo actuator drives an hydraulic steering valve 92 of thesteering control means 24 associated with the aircraft nose-wheel tosteer the aircraft back on to the path axis 14. In addition, the servoactuator 90 controls the tap on the slide wire 86 so as to continuouslygenerate the signal -0 in the form of a feedback signal.

The expression 0,, lK,D,,l K D has been relied upon in the foregoing. Ithas been found to represent a conservative and convenient expression forthe computation of 0, where K da /(1D,, (2D,,, i ,,,)/D,, 0,,,,,/2D,,,q5,,, l/V,, and 0, is the maximum or steepest a proach angle whichmay be expressed as 0,, 2D,, /V where equals maximum nose wheel turningrate in radians per second, V equals aircraft velocity in feet persecond, and V,, is the maximum for the velocity V.

Parallax correction is performed by the amplifier 78 mentionedpreviously. In this connection, it will be noted that the parallaxcorrection is performed in accordance with the expression D, D, Xsin 0,.Of course the value X0, may be approximated by the expression sin X0,for small angles of 0,. Accordingly, to a very good approximation, D D,X0,,.

In the embodiment described in the foregoing, a two wire track isutilized, each wire lying on either side of the predetermined path axis14. As mentioned previously, the two-wire system requires a constantlevel field. In the embodiment now to be described with reference toFIG. 5 and 6, a single wire track is utilized with the single wireparallel with the path axis 14. This system does not require a constantlevel field.

As shown in FlG. 5, the ground track sub-system comprises a singleoscillator means generating a signal F=I sin wt where w=21lf and f mayequal 1,000 Hz. The signal from the oscillator 100 is applied to thepower amplifier 102 and coupled to a single wire track 104. The circuitis completed through a shielded wire 106.

As in the case of a two wire sub-system, the pick-up coils 16-1 and 16-2are utilized to sense the horizontal components of the field generatedby the wire 104 as shown in FIG. 5. in addition, a field gradientsensing coil 108 is utilized in the vehicle sub-system. The horizontalfield components which are sensed by the coil 108 induce a signalincluding a component F(D, cos 0,) which is applied to an amplifier 110,an active filter 112, and an AC to DC converter 114 including anamplifier 115 forming part of a ll network filter to obtain a signal D,cos 0,. Similarly, the signal including a component F(D, cos 0,) inducedin the coil 16-1 is applied to an amplifier 116, an active filter 118,and an AC to DC converter including an amplifier 122 forming partdf'a'il hatv'a'riz T14 to asai'aasigaarb. cos 0,. M

The output of the converters 114 and are applied to a differentialamplifier 124 to obtain a signal representing the field gradient betweenthe pick-up coils 16-1 and 108 expressed as AD,/d where d is theseparation distance (center to center) between the pick-up coil 108 andthe pick-up coil 16-1 and D, is the separation distance between thecoils 16-1 and the single wire 104 which is parallel with thepredetermined path axis 14. By comparing the output of the amplifier 124with a predetermined threshold voltage at an amplifier 126, thethreshold voltage being provided by a tap on a potentiometer 128, an inarea signal is obtained. The in area signal may then be utilized toactivate the automatic guidance system through a switch (electronic orotherwise). With this arrangement, the automatic guidance system willmaintain control only within the following limits: ID, cos 0,,(108)|i|D, cos 0,,(16-1) l e.

The signal including a component F(D, sin 0,) induced in the pick-upcoil 16-2 is amplified by an amplifier 134, filtered by an active filter136 and then demodulated by a phase demodulator 138. The signal F D, sin0,) has a phase characteristic which depends on which quadrant the angle0,, is in. The direction and magnitude of this angle is detected by thephase demodulator 138 whose reference signal phase is F(D, cos 0,) andamplified at an amplifier 140. The signal F(D, sin 0,) is then appliedto a low pass filter 142.

Both the output signal F (D, cos 0,) of the converter 120 and the outputsignal F(D, sin 0,) of the low pass filter 142 are now applied todividers 144 and 146 respectively along with a field gradient signal forpurposes of linearization. Although the field is relatively linear in atwo wire guidance track under certain constraints of path width versuscoil height, the field is relatively non-linear in a one wire guidancetrack system. This non-linear characteristic can be minimized bynormalizing the value of D, for a given operating domain by dividing theF(D,) by the field gradient. This division is performed by a dividers144 and 146 in combination with potentiometers 148 and 150 having thefield gradient signal AD,/d coupled thereto. After linearization, thesignal KD, sin 0,, is obtained at the output of the divider 146.Similarly, a signal KD cos 0,, is obtained at the output of the divider144.

The output of the divider 146 provides the signal which may be utilizedin the approximation of 0,, for solution of the steering equation at adifferential amplifier 150. However, the angle 0,, must be obtained byfirst determining the value of D,. The value of; D, is derived from thetwo linearized signals KD sin 0,, and KD, cos 0,, by the use of a vectoroperator module 152 having an output function equal to the square rootof X Y so as to operate upon the K0, sin 0,, and KD, cos 0,, signals inthe following manner: \/(D, sin 0,) (D, cos 0,) |D,| 0,, may now becomputed from |D,| Note that the determination of D is independent ofthe vehicle angle 0,.

The |D,,| signal is first applied to a buffer amplifier 154. An RC rateadjust network 156 is provided to obtain the signal representing |K D,|while a square root module 158 is provided to obtain a signalrepresenting |K,D,| After subtraction is performed at an amplifier 160,a signal representing 0,, is obtained and applied to a multiplier 162along with the |D,l signal. Note that a potentiometer 168 has beenprovided in the circuitry of the summing amplifier 160. Thispotentiometer provides a centering set point which determines thetracking path of the pick-up coils 16-1 with respect to the single wire104.

The l), signal is also applied to an inverter 164 having an outputproviding a power supply for a slide wire 166. By controlling theposition of the tap of the slide wire 168 in response to the position ofthe servo actuator 90,

a feedback signal representing the nose wheel angle or steered path 4)is obtained for purposes of solving the steering equation (0,, 0,,) 0.The signals representing 0,, 0,, and (ii are now applied to thedifferential amplifier to obtain an error or deviation signal whichserves as the input to the steering control means 20.

The deviation computer in the foregoing specification has been describedin analog form. It will be appre ciated by those of ordinary skill inthe art that a special purpose digital computer which operates in realtime might be utilized to determine the necessary quantities for solvingthe steering equation. Other modifications may also occur to those ofordinary skill in the art. The appended claims are intended to coversuch modifications which fall within the spirit and scope of theinvention.

What is claimed:

1. In a system for guiding a vehicle having a steering means along apredetermined path on a surface, wherein an angle 0,, represents theangle between the predetermined path and the path steered by saidsteering means, an angle 0,, represents the angle between thepredetermined-path and a longitudinal axis of the vehicle, and an angle4) represents the angle between the steered path and the longitudinalaxis of the vehicle, said system comprising:

a track means for generating a field along said predetermined path, saidtrack means being fixed with respect to said surface;

a pick-up means carried by said vehicle for sensing said field;

a deviation computer means carried by said vehicle for solving asteering equation (1) 0,, 0,, comprising means for determining theangles 0,, and 0,, in response to said sensed field and means fordetermining the difference between the angle 5 and the sum of the angles0,, and 0,; and

a steering control means carried by said vehicle for controlling saidsteering means in response to said difference in a manner tending toreduce said difference.

2. The system of claim 1 wherein said means for determining the angle0,, comprises:

a means for determining (K,D)" in response to said sensed field where Dis the distance between said steering means and said predetermined pathand K isa constant;

a means for determining |K Dl in response to said sensed field where Kis a constant; and

a means for determining the difference between (IK D W and (KzD), saiddifference approximating 9".

3. The system of claim 2 wherein, K, 8 {p /V and K -l/V,,, where (22,,is the maximum turning rate of the steering means and V,,, is themaximum velocity of the vehicle.

4. The system of claim 3 whwerein said means for determined path and atleast one oscillator means coupled to said at least one wire.

7. The system of claim 6 wherein said track means comprises a singlewire and a single oscillator and said sensing means further comprisesanother pick-up coil having an axis perpendicular to the longitudinalaxis of the vehicle and a means for determining the field gradient ADJdbetween said other pick-up coil and the longitudinal axis of thevehicle, where d is the separation distance between said other pick-upcoil and said one of said pair of said coils having an axis parallel tothe longitudinal axis of the vehicle and D, is the distance separatingsaid one of said pair of coils and said single wire so as to indicatethe distance of the vehicle from said single wire.

8. The system of claim 7 wherein said deviation computer means includes:

a means for normalizing the value of D, by dividing a signalrepresenting a function of D, by said field gradient AD ld.

9. The system of claim 8 wherein said deviation computer means includesmeans for generating an in area signal from said field gradient.

10. A vehicle guidance system comprising:

at least one reference signal generating means;

at least one track wire defining a predetermined path of travel for saidvehicle over a surface, said reference signal generating means beingcoupled to said track wire to generate an electromagnetic field alongsaid predetermined path;

a pair of pick-up coils mounted on said vehicle, said coils havingmutually orthogonal axes lying in a plane substantially parallel to saidsurface for sensing said electromagnetic field;

deviation determining means coupled to both said coils and responsive tothe electromagnetic field sensed thereby for determining the angulardeviation of said vehicle from said predetermined path and the lateraldisplacement from said predetermined path independent of theinstantaneous steering angle of said vehicle with respect to saidpredetermined path; and

steering means responsive to the output of said deviation determiningmeans.

11. The vehicle guidance system of claim 10 including feedback meanscoupled between said steering means and said deviation determiningmeans.

12. The vehicle guidance system of claim 11 wherein said deviationdetermining means comprises a means for determining the angle 6,,representing the angle be tween the predetermined path and the steeredpath of the steering means, a means for determining the angle 6,,representing the angle between the predetermined path and thelongitudinal axis of said vehicle, and a means for determining the angleqb representing the angle between the steered path of said steeringmeans and longitudinal axis of the vehicle, said steering control meanscontrolling said steering means in a manner tending to maintain equalitybetween the angle 42 and the sum of the angle 0,, and 0 13. The taxiguidance system of claim 12 wherein said system includes a pair of trackwires defining said predetermined path of travel, said reference signalgenerating means coupled to said pair of track wires, one

of said pair of track wires carrying a signal representing the sum oftwo sinusoidal functions of different frequencies provided by saidsignal generating means and the other of said track wires carrying asignal representing the difference of said two sinusoidal functions asprovided by said signal generating means.

14. The taxi guidance system of claim 12 comprising a single track wirecarrying a signal representing a sinusoidal function of a singlefrequency generated by said generating means.

15. The taxi guidance system of claim 12 wherein said deviationdetermining means includes ameans of determining the angle 0,, without aconstant level field.

1. In a system for guiding a vehicle having a steering means along apredetermined path on a surface, wherein an angle theta a represents theangle between the predetermined path and the path steered by saidsteering means, an angle theta p represents the angle between thepredetermined path and a longitudinal axis of the vehicle, and an anglephi represents the angle between the steered path and the longitudinalaxis of the vehicle, said system comprising: a track means forgenerating a field along said predetermined path, said track means beingfixed with respect to said surface; a pick-up means carried by saidvehicle for sensing said field; a deviation computer means carried bysaid vehicle for solving a steering equation phi theta a + theta pcomprising means for determining the angles theta a and theta p inresponse to said sensed field and means for determining the differencebetween the angle phi and the sum of the angles theta a and theta p; anda steering control means carried by said vehicle for controlling saidsteering means in response to said difference in a manner tending toreduce said difference.
 2. The system of claim 1 wherein said means fordetermining the angle theta a comprises: a means for determining(K1D)1/2 in response to said sensed field where D is the distancebetween said steering means and said predetermined path and K1 is aconstant; a means for determining ( K2D ) in response to said sensedfield where K2 is a constant; and a means for determining the differencebetween ( K1D )1/2 and (K2D), said difference approximating theta a. 3.The system of claim 2 wherein, K1 8 phi m/Vm and K2 -1/Vm where phi m isthe maximum turning rate of the steering means and Vm is the maximumvelocity of the vehicle.
 4. The system of claim 3 whwerein said meansfor determining the angle theta p determines the sin theta p as anapproximation of the angle theta p.
 5. The system of claim 1 whereinsaid sensing means comprises a pair of pick-up coils hAving mutuallyorthogonal axes lying in a plane parallel to said surface, one of saidpick-up coils having an axis parallel to the longitudinal axis of saidvehicle.
 6. The system of claim 5 wherein said track means comprises atleast one wire extending along said predetermined path and at least oneoscillator means coupled to said at least one wire.
 7. The system ofclaim 6 wherein said track means comprises a single wire and a singleoscillator and said sensing means further comprises another pick-up coilhaving an axis perpendicular to the longitudinal axis of the vehicle anda means for determining the field gradient Delta Ds/d between said otherpick-up coil and the longitudinal axis of the vehicle, where d is theseparation distance between said other pick-up coil and said one of saidpair of said coils having an axis parallel to the longitudinal axis ofthe vehicle and Ds is the distance separating said one of said pair ofcoils and said single wire so as to indicate the distance of the vehiclefrom said single wire.
 8. The system of claim 7 wherein said deviationcomputer means includes: a means for normalizing the value of Ds bydividing a signal representing a function of Ds by said field gradientDelta Ds/d.
 9. The system of claim 8 wherein said deviation computermeans includes means for generating an ''''in area'''' signal from saidfield gradient.
 10. A vehicle guidance system comprising: at least onereference signal generating means; at least one track wire defining apredetermined path of travel for said vehicle over a surface, saidreference signal generating means being coupled to said track wire togenerate an electromagnetic field along said predetermined path; a pairof pick-up coils mounted on said vehicle, said coils having mutuallyorthogonal axes lying in a plane substantially parallel to said surfacefor sensing said electromagnetic field; deviation determining meanscoupled to both said coils and responsive to the electromagnetic fieldsensed thereby for determining the angular deviation of said vehiclefrom said predetermined path and the lateral displacement from saidpredetermined path independent of the instantaneous steering angle ofsaid vehicle with respect to said predetermined path; and steering meansresponsive to the output of said deviation determining means.
 11. Thevehicle guidance system of claim 10 including feedback means coupledbetween said steering means and said deviation determining means. 12.The vehicle guidance system of claim 11 wherein said deviationdetermining means comprises a means for determining the angle theta arepresenting the angle between the predetermined path and the steeredpath of the steering means, a means for determining the angle theta prepresenting the angle between the predetermined path and thelongitudinal axis of said vehicle, and a means for determining the anglephi representing the angle between the steered path of said steeringmeans and longitudinal axis of the vehicle, said steering control meanscontrolling said steering means in a manner tending to maintain equalitybetween the angle phi and the sum of the angle theta a and theta p. 13.The taxi guidance system of claim 12 wherein said system includes a pairof track wires defining said predetermined path of travel, saidreference signal generating means coupled to said pair of track wires,one of said pair of track wires carrying a signal representing the sumof two sinusoidal functions of different frequencies provided by saidsignal generating means and the other of said track wires carrying asignal representing the difference of said two sinusoidal functions asprovided by said signal generating means.
 14. The taxi guidance systemof claim 12 comprising a single track wire carrying a signalrepresenting a sinusoidal function of a single frequency generated bysaid generating meAns.
 15. The taxi guidance system of claim 12 whereinsaid deviation determining means includes a means of determining theangle theta p without a constant level field.